The uterus needs to repair itself prior to hosting another fetus and the girl needs to cycle early. I would ensure body condition going into calving and then ensure she has adequate to plenty of protein in her diet during the first 30 to 40 days postpartum as tissue repair uses proteins as well as requiring additional energy. Until the uterus is repaired, they won't get pregnant and carry to term.
I would pull the calves off the girls and then keep them off for three days, when you want them to begin their cycle. The cow won't dry up and this will sometimes trigger a heat cycle. I second earlier lute comments.
AI or NS, use a calving ease sire with a lower birth weight. Our experience is calves out of calving ease sires tend to drop up to 10 days earlier. Calf is generally smaller and stresses the girl less.
If these are heifers or there are any issues with a difficult birth in January I wouldn't consider moving them back significantly next breeding.
